1. Field of the Invention
The present invention relates to a method for reducing emission of pollutants from an internal combustion engine, particularly from a diesel engine, and to a fuel emulsion comprising water and a liquid hydrocarbon.
1. Description of the Related Art
It is known that the combustion of liquid hydrocarbons in an internal combustion engine (e.g. a diesel engine) leads to the formation of numerous pollutants, in particular soot, particulates, carbon monoxide (CO), nitrogen oxides (NOx), sulphur oxides (SOx), and non-combusted hydrocarbons (HC), which cause a remarkable atmospheric pollution.
It is also known that the addition of controlled amounts of water to a fuel can significantly reduce the production of pollutants. It is believed that this effect is the result of various phenomena arising from the presence of water in the combustion zone. For example, the lowering of the peak combustion temperature by water reduces the emission of nitrogen oxides (NOx), the formation of which is promoted by high temperatures. In addition, the instantaneous vaporization of the water droplets promotes better dispersion of the fuel in the combustion chamber, thereby significantly reducing the formation of soot, particulates and CO. These phenomena take place without adversely affecting the yield for the combustion process.
Several solution have been proposed to add water to liquid fuel at the time of use, i.e. just before the fuel is injected into the combustion chamber, or directly into the chamber itself. However, these solutions require modifications to be made to the structure of the engine and are not capable of achieving optimum dispersion of the water in the fuel, which is an essential requisite for obtaining a significant reduction in pollutants without compromising the calorific yield for the process.
Thus, the most promising and numerous efforts made hitherto were directed towards the formulation of emulsions between liquid hydrocarbons and water in the presence of emulsifiers (surfactants) for the purpose of uniformly dispersing the water in the hydrocarbon phase in the form of droplets of the smallest possible size.
For example, European Patent Application EP-A-475,620 describes microemulsions of a diesel fuel with water, which contain a cetane improver and an emulsifying system comprising a hydrophilic surfactant and a lipophilic surfactant. These surfactants are selected from ethoxylated C12-C18 alkylammonium salts of a C9-C24 carboxylic or sulphonic acid: the hydrophilic surfactant contains at least six ethylene oxide units, while the lipophilic surfactant contains less than six ethylene oxide units.
European Patent Application EP-A-630,398 describes a fuel in the form of an emulsion consisting of a hydrocarbon fuel, from 3 to 35% by weight of water and at least 0.1% by weight of an emulsifying system consisting of a sorbitan oleate, a polyalkylene glycol and an ethoxylated alkylphenol.
International Patent Application WO 97/34969 describes an emulsion between water and a hydrocarbon, for example a diesel fuel. This emulsion is stabilized by adding an emulsifier consisting of a sorbitan sesquioleate, a polyethylene glycol monooleate and an ethoxylated nonylphenol. This emulsifier has an overall HLB (hydrophilic-lipophilic balance) value of from 6 to 8.
A process for producing a stabilized emulsion of a liquid fuel and water is described in European Patent Application EP-A-812,615. This process involves preparing a first emulsion by mixing the fuel, the water and a surfactant, and subsequently mixing the emulsion thus obtained with more water to give the final emulsion. The emulsion is stabilized using a hydrophilic surfactant or a lipophilic surfactant, or a mixture thereof. Lipophilic surfactants which can be used are fatty acid esters of sorbitol, for example sorbitan monooleate, while hydrophilic surfactants which are suitable for this purpose are fatty acid esters of sorbitol containing a polyoxyalkylene chain, for example polyoxyethylene sorbitan trioleate. Further stabilization of the emulsion can be obtained by adding ethylene glycol or a polyethylene glycol.
International Patent Application WO 92/19701 describes a process for reducing the emission of NOx from a gas turbine, in which an emulsion of water with a diesel fuel is used. The emulsion is stabilized by adding an emulsifier selected from: alkanolamides obtained by condensing an alkylamine or hydroxyalkylamine with a fatty acid; and ethoxylated alkylphenols. The emulsifier preferably has a HLB value of less than or equal to 8. Physical stabilizers such as waxes, cellulose derivatives or resins can be added to improve the stability. As described in patent application WO 93/07238, the above emulsion can be further stabilized by adding a difunctional block polymer with a primary hydroxyl end group, in particular a copolymer containing propylene oxide/ethylene oxide blocks.
International Patent Application WO 00/15740 describes an emulsified water-blended fuel composition comprising: (A) a hydrocarbon boiling in the gasoline or diesel range; (B) water; (C) a minor emulsifying amount of at least one fuel-soluble salt made by reacting (C) (I) at least one acylating agent having about 16 to 500 carbon atoms with (C) (II) ammonia and/or at least one amine; and (D) about 0.001 to about 15% by weight of the water-blended fuel composition of a water soluble, ashless, halogen-, boron-, and phosphorus-free, amine salt, distinct from component (C). The acylating agent (C) (I) includes carboxylic acids and their reactive equivalents such as acid halides, anhydrides, and esters, including partial esters and triglycerides. The fuel may also comprise other components such as: cosurfactants selected from ionic or non-ionic compounds having a HLB of from 2 to 10, preferably of from 4 to 8; organic cetane improvers, including nitrate esters of substituted or unsubstituted aliphatic or cycloaliphatic alcohols; antifreeze agents, usually an alcohol such as ethylene glycol, propylene glycol, methanol, ethanol, and mixtures thereof, in a an amount of from 0.1% to 10%, preferably from 0.1 to 5%, by weight of the fuel composition.
International Patent Application WO 01/51593 describes a fuel comprising an emulsion between water and a liquid hydrocarbon, and further comprising as emulsifier a polymeric surfactant obtainable by reaction between: (i) a polyolefin oligomer functionalized with at least one group deriving from a dicarboxylic acid, or a derivative thereof; and (ii) a polyoxyalkylene comprising linear oxyalkylene units, said polyoxyalkylene being linked to a long-chain alkyl group optionally containing one or more ethylenic unsaturations. The fuel may also comprise an alcohol as antifreeze agent, such as methanol, ethanol, isopropanol, or a glycol, in an amount generally from 0.5 to 8% by weight, preferably from 1 to 4% by weight, with respect to the total weight of the fuel.
A reduction of NOx exhaust emissions from a diesel engine can also be obtained by controlling the functioning of the engine so as to obtain a reduction of the peak combustion temperature.
Such a reduction may be obtained for instance by recirculation of a portion of the exhaust gases into the engine intake manifold where it mixes with the incoming air/fuel charge. By diluting the air/fuel mixture under these conditions, peak combustion temperatures are reduced, resulting in an overall reduction of NOx output. Such systems are commonly known as Exhaust Gas Recirculation (EGR) systems. The first EGR systems were introduced in the early '70s as on/off devices. However, continuous recirculation of the exhaust gases resulted in unstable engine operation, decreased power output and oil contamination due to the presence of particulates in the recirculated gases. Upon introduction of close loop computer controls for engines, the EGR systems were remarkably improved by controlling the rate or amount of recirculated exhaust gases in a manner responsive to operating conditions of the engine, particularly-during acceleration. For a general review on EGR systems see for instance “Emission Controls: Part II: GM Exhaust Gas Recirculation Systems” by M. Schultz, published in Motor, Vol. 159 (February 1983), pages 15 ff, and also U.S. Pat. Nos. 3,796,049 and 4,454,854.
Another system for reducing the peak p combustion temperature, and thus the NOx emissions, by controlling the functioning of the engine is based on an electronic control of the injection timing in the combustion chamber. Particularly, delayed injection reduces NOx emissions, while excessive delay results in higher fuel consumption and HC emissions. Therefore, a precise injection timing is necessary, which is guaranteed by an electronic diesel-control system (EDC). A crankshaft reference point provides the basis for regulating the timing device setting. Extremely high precision can be achieved by monitoring the start of injection directly at the injection nozzle by employing a needle-motion sensor to monitor the needle-valve movement (control of start of injection) (see for instance U.S. Pat. No. 5,445,128).
Another known method to reduce NOx in exhaust gases is based on cooling compressed intake air in turbocharged engines, so as to reduce combustion temperatures in the engine, with a consequent decrease of NOx emissions. A method of this kind is disclosed for instance in U.S. Pat. No. 6,145,498.
For a general review on engine measures to reduce exhaust emissions from diesel engines see for instance “Bosch Automotive Handbook”, 4th Edition, October 1996 (pages 530-535).
In order to meet the requirements of increasingly more stringent emission standards, some attempts have been made to combine different technologies of emission reduction.
For instance, U.S. Pat. No. 4,479,473 a system for controlling emissions from a diesel engine is disclosed by controlling the recirculation of engine exhaust gases into the intake manifold and by modulating the injection timing schedule of the engine fuel injection pump.
U.S. Pat. No. 5,271,370 discloses an emulsion fuel engine having at least one cylinder with an injection nozzle for injecting an emulsion fuel, which has been formed by mixing a first fuel with a second fuel, into the cylinder. The engine comprises exhaust gas recirculation means for returning a portion of exhaust gas to an intake passage to recirculate the exhaust gas; and exhaust gas recirculation control means for controlling the amount of the exhaust gas to be recirculated. Therefore, water and diesel fuel are mixed for the first time when the engine is operated by the emulsion fuel. Alternatively, an emulsion fuel prepared in advance by mixing diesel fuel and water and stored in an emulsion fuel tank can be delivered to the injection nozzle and then injected into the cylinder.